In a networked cluster, the network clients, or nodes, communicate over paths established between pairs of nodes. In such an environment, a message may be lost when a fault is encountered in the path of the message. A reliable network has the capability to take appropriate action to guarantee delivery. However, when a network does not have the capability to guarantee delivery or detect delivery failure, then the network is unreliable.
When links fail in a network, and network interfaces continue to send message packets over these failed links, the applications running on the hosts will experience packet drops. For example, if there are four routes available between a source-destination node-pair, and one of them has a faulty link, then the applications will see a twenty-five percent packet loss. If such packets are part of a relatively large message, then the probability of the entire message being successfully received decreases to nearly zero. A traditional solution for this problem is to replace a failing route with a good route. Using this approach, the original route needs to be replaced when the failed link becomes operational again. Otherwise, over time, the static balance of routes in the network will suffer degradation.
In an unreliable network, clients are typically required to retransmit messages until they are delivered successfully. This direct approach, i.e., of retransmitting lost messages, is generally not acceptable in high performance environments. For example, one such environment is a clustered supercomputer in which a number of servers are connected together using a high-speed network. These servers rely on fast, reliable message transfer for application efficiency.
Accordingly, there is a need to overcome the aforementioned drawbacks of an unreliable network in order to enable reliable, fast delivery of messages in a networked cluster.